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Boyle's Law Prelab Boyle’s Law Prelab by Dr. Christine P. Cheney, Department of Physics and Astronomy, 401 Nielsen Physics Building, The University of Tennessee, Knoxville, Tennessee 37996-1200 © 2018 by Christine P. Cheney* *All rights are reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage or retrieval system, without the permission in writing from the author. Read the Boyle’s Law introduction in your lab manual.1 Boyle’s Law originates from the ideal gas law (Equation 1 in your manual) where the temperature is kept constant. When pressure is increased on an ideal gas at a constant temperature, then the volume is decreased (P1V1=P2V2). In the Boyle’s Law lab, you will increase the pressure on a piston by adding mass to it. You will then measure the change in volume of the gas by measuring the change in height of the piston (which is a syringe in our case). From your measurements you will extract the atmospheric pressure in the lab. Boyle’s Law applies to compressible fluids. Air is a compressible fluid whereas water is not. Water has almost no change in volume as its pressure increases. A gas will expand as its altitude goes up since there is less pressure on it from the surrounding atmosphere. A gas will contract as the altitude goes down or if it is taken under water (about every 10 m of water is equivalent to 1 atm pressure). Scuba divers know that they have to ascend to the surface of the water slowly after being deep underwater. When a diver is deep underwater, pressure will increase and gas volumes will decrease.2 As the diver descends, more nitrogen and oxygen gases dissolve in the blood.3 Most of the oxygen gets used by your body tissue, but the nitrogen remains dissolved. When the diver ascends, the gas will expand since the pressure decreases. Nitrogen gases are released from the blood. If the ascent happens too quickly, the gases do not have enough time to be released out of the blood. The gases come out of solution as bubbles in the blood. These bubbles can block tiny blood vessels. An air embolism occurs when air bubbles enter a vein (veinous air embolism) or artery (arterial air embolism) and block it.4 These air bubbles can travel to your brain, heart, or lungs and cause a heart attack, stroke, or respiratory failure. Thus, too quick of an ascent can kill a person. Nitrogen will also expand at the joints and cause the diver to bend over in pain (get the bends or decompression sickness).2, 5 A scuba diver also knows that they need to breathe out as they ascend (gas expanding in lungs) and not hold their breath. If they take a breath at 30 m under water where the pressure is about 4 atm (1 atm pressure from the air + 3 atm pressure of water) and ascend without breathing out, the volume in their lungs will quadruple when they reach the surface at 1 atm pressure.6 Air medical transport has to think about the patient and the devices aiding the patient to prevent further injury in the event of decompression of the aircraft.7 For example, an air splint will increase in volume with decreasing pressure. Air in IV bags will expand and have the potential to cause an air embolism in the patient. Questions: 1. What is the volume of a lung at 3 atm pressure if its volume is 1 liter at 1 atm pressure? 2. A balloon contains 6.2 l of helium. The pressure is reduced to 2.00 atm and the balloon expands to occupy a volume of 26.1 l. What was the initial pressure exerted on the balloon? 3. How will you find the frictional force in the Boyle’s Law experiment? References: 1 Parks, James E. Contemporary Introductory Physics Experiments, 2nd Edition, Hayden-McNeil Publishing: Plymouth 2014. 2 https://owlcation.com/stem/Examples-of-Boyles-Law 3 https://adventure.howstuffworks.com/outdoor-activities/water-sports/scuba3.htm 4 https://www.healthline.com/health/air-embolism 5 https://prezi.com/09vux96lbpo6/the-bends-and-boyles-law/ 6 https://www.carolina.com/teacher-resources/Interactive/scuba-diving-and-gas- laws/tr29802.tr 7 http://dhss.alaska.gov/dph/Emergency/Documents/ems/assets/AirMedCourse/EMS- E_Chapter3.pdf .
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